Java solution to the string decoding problem: https://leetcode.com/problems/decode-string/

StringDecode.java

import java.util.function.Predicate;
import java.util.stream.Stream;

public class Solution {

    private int indexOfPred(String s, Predicate<Character> p) {
        for (int i = 0; i < s.length(); i++) {
            if (p.test(s.charAt(i))) {
                return i;
            }
        }
        return -1;
    }

    private int endIndexOfGroup(String s) {
        assert s.charAt(0) == '[';

        int depth = 1;
        for (int i = 1; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '[') depth += 1;
            else if (c == ']') depth -= 1;

            if (depth == 0) {
                return i;
            }
        }
        throw new RuntimeException("Unmatching end parentheses in string: " + s);
    }

    private String repeatString(int count, String decodedParen) {
        return Stream.generate(() -> decodedParen)
                .limit(count)
                .reduce("", String::concat);
    }

    public String decodeString(String s) {
        if (s.isEmpty()) {
            return "";
        }

        if (Character.isDigit(s.charAt(0))) {
            int parenIndex = s.indexOf('[');
            String digits = s.substring(0, parenIndex);
            int count = Integer.parseInt(digits);

            String parenAndRest = s.substring(parenIndex);
            int endIndex = endIndexOfGroup(parenAndRest);
            String parenUnpacked = parenAndRest.substring(1, endIndex);
            String rest = parenAndRest.substring(endIndex + 1);

            String decodedParen = decodeString(parenUnpacked);

            String decodedFirst = repeatString(count, decodedParen);
            return decodedFirst + decodeString(rest);
        } else {
            int firstNonLetter = indexOfPred(s, c -> !Character.isLetter(c));
            if (firstNonLetter >= 0) {
                String allLetters = s.substring(0, firstNonLetter);
                String rest = s.substring(firstNonLetter);

                return allLetters + decodeString(rest);
            } else {
                return s;
            }
        }
    }

    public static void main(String[] args) {
        Solution s = new Solution();
        System.out.println(s.decodeString("3[a]2[bc]"));
        System.out.println(s.decodeString("3[a2[c]]"));
        System.out.println(s.decodeString("2[abc]3[cd]ef"));
        System.out.println(s.decodeString("100[leetcode]"));
    }
}